NECESSARY EQUIPMENT
The main considerations with regard to the equipment
necessary for the use of Differential Global Positioning Systems
are the type of GPS receiver and the type and location of the differential
station. There are several brands of receivers available and the technology
is being updated rapidly and continuously. For use in the context
of commercial salvage on the Florida East Coast Shipwreck Project, it is
important to take cost into consideration, and to balance that with accuracy.
As far as GPS receivers are concerned, the majority of errors are caused
by noise, the improper implementation of DGPS algorithms, and the antenna
performance with regard to multipath errors (reflections of signals that
have bounced off surrounding objects and are picked up by the receiver
antenna). By choosing a receiver with low noise, ensuring the proper
implementation of the algorithms, as well as proper antenna installation
and position fix averaging, the majority of these errors can be minimized
(Weber and Tiwari, no date). Another consideration with the receivers is
the number of decimal places they are capable of reading to in the seconds
column. Although a receiver may be capable of submeter accuracy, if it
cannot read out to at least two decimal places in the seconds column, then
the unit will not be capable of relaying the information to the degree
of submeter accuracy (Two decimal places in the seconds column is equivalent
to 1 foot or .31 meters, one decimal place in the seconds column is equivalent
to 10 feet or 3 meters).
By taking into consideration the cost of various GPS units, the most cost efficient and accurate unit available under these circumstances is the Garmin 45 XL. It advertises accuracy to the degree of 5-10 meters with differential corrections, and is capable of relaying information to the three meter level of accuracy. The cost of the unit itself is $279.00 and the Garmin GBR 21 differential receiver antenna is $469.00. The differential receiver antenna is set to receive signals from the Coast Guard’s Continuously Operating Reference Stations or CORS, the closest of these being the one at Cape Canaveral, Florida. There are, of course, more accurate systems available; however, the cost of such systems jumps significantly from the approximately $800 for the system mentioned above to $5000 for a Starlink system, and up from there to $20,000+ for even more sophisticated systems such as the Topscon system used by the State of Florida for positioning the beach markers in 1996. Due to the exorbitant cost of more accurate systems, Salvors, Inc. feels it is reasonable to suggest the use of the Garmin 45 XL with the GBR 21 differential receiver. This equipment will provide a satisfactory level of accuracy at a manageable price.
DIFFERENTIAL SIGNAL BROADCAST SITES
The choice of a broadcast site for the differential
signal is extremely important with regard to this issue. For the area in
which Salvors, Inc. and its subcontractors operate, there are basically
two choices. The first of these is the Differential Corrections, Inc. (DCI)
FM signal which offers three levels of service: 1) The Premium One
Meter service for $600 per year, 2) The Intermediate Five Meter service
for $250 per year, and 3) The Basic Ten Meter service for $75 per year.
Salvors, Inc. has tested this system at the rate of the Premium Service
for the past year and a half. The accuracy appears to be what is
advertised; however, there is one major problem with this service within
the context of this project. The DCI FM signal cannot reach as far as Fort
Pierce Inlet, thus making it impossible for the subcontractors working
the 1810 site and the Douglas Beach sites to receive differential corrections.
As these sites are worked regularly throughout the course of a season,
this is not satisfactory as a sole means of positioning. Additionally,
there is no yearly charge for the second option, the Coast Guard’s Continuously
Operating Reference Stations or CORS.
The closest of the CORS to the area being discussed is the station at Cape Canaveral. This station has a signal strength of 200 nautical miles, and after purchasing a differential receiver antenna geared for this signal, the subcontractors, namely John Brandon, working the Douglas Beach sites were able to receive it without any problems. The CORS do not have different levels of service as in the case of DCI; however, Lieutenant Commander Gary Schenk of the DGPS Operations Management Division for the Coast Guard stated that the accuracy of the signal being broadcast from Cape Canaveral is 2.5 meters or better 95% of the time for areas within 100 miles of the station, with that number (2.5 meters) being increased by 1 meter for every 100 miles thereafter. He further stated that new technology, already in use at Egmont Key, will be available at the Cape in the future, thereby increasing that accuracy to 1 meter 95% of the time for areas within 100 miles of the station. Additionally, he noted that the only disruption of the signal that may occur is a delay in the update rate from 5-6 seconds to 15 seconds at 100 bytes per second as a result of thunderstorm activity (Schenk, personal communication 1997). Within the context of the type of activity on the 1715 fleet, this should not affect the subcontractors as they take their positions.
CONCLUSION
The signal and accuracy from the Cape Canaveral
Station is compatible with the accuracy level of the Garmin 45 XL and the
GBR 21 antenna that Salvors, Inc. is recommending for use on the Florida
East Coast Shipwreck Project. Additionally, the system comes with a PC
kit which Salvors, Inc. already has that enables the information from each
of the individual units to be downloaded into the main computer located
at the laboratory at 1322 US 1. This will enable Salvors, Inc. to transfer
the data into a database which can then be sent to the State of Florida
Division of Historical Resources. The State officials can then transfer
the data into their plotting programs in order to create a map of the areas
worked for the season. This feature will allow for the elimination of the
basemaps. The subcontractors can keep track of the areas they have worked
by entering them as waypoints which will then be downloaded into the main
computer. The subcontractors will continue to maintain logsheets for the
purposes of tracking artifacts found and their tag numbers as well as having
a hard copy of their locations in the event of a system failure, but they
should no longer have to plot their locations on a basemap using this system.
They may wish to obtain copies of basemaps to plot locations for their
own personal use; however, it should no longer be necessary for the
purposes of the State. Thus, this system is not only of benefit to position
location, it will also help to eliminate paperwork, and the errors that
may occur within the individual plotting of basemaps. Furthermore, access
to this data will be far more efficient for both the State and Salvors,
Inc. The Coast Guard stations do not charge a yearly service fee,
the subcontractors will not have to worry about additional yearly costs.
It is the feeling of Salvors, Inc. that the options discussed in this section
represent a fair and cost efficient means of changing to DGPS as
a method of position location. The use of the Garmin equipment and the
CORS for a differential corrections signal will allow for a satisfactory
degree of accuracy (three meters or 9.84 feet 95% of the time and no more
than 10 meters or 32.8 feet the rest of the time) that will ultimately
provide far more reliable data than the previous method of beach markers
and sextant readings. Salvors, Inc. is confident that this method
will improve the quality of the work done on the Florida East Coast Shipwreck
Project.